The climate control systems for passenger compartments of vehicles often have a limited ability to maintain the comfort of the vehicle passengers. One of the difficult to eliminate factors which causes passenger discomfort is the presence of unwanted drafts in the vehicle. By design, most passenger compartments of vehicles are not air tight. As a result, outside air is able to infiltrate the vehicle through various leakage points and cause drafts, especially when the vehicle is traveling at highway speeds. These drafts may cause passenger discomfort and interfere with the desired circulation patterns of the climate control system. Although the drafts are most often felt when the vehicle is traveling at high speeds, high velocity winds may cause drafts when the vehicle is standing still.
One known method to solve the problem of drafts in the vehicle is to pressurize the passenger compartment. One example is disclosed in the U.S. Pat. No. 4,788,903, to Johnson issued Dec. 6, 1988, and assigned to the assignee of the present invention. The intent is to maintain a constant positive pressure, relative to the outside ambient pressure. As long as a positive pressure is maintained inside the passenger compartment of the vehicle, the infiltration of drafts will be minimized. However, the previous solutions are limited in several respects. First, their ability to maintain a substantially constant pressure is limited because of the simplicity of the control structure and the large number of unaccounted for variables in the system. A second limitation is the inability to accurately determine the outside ambient air pressure. A third limitation is the inability to control the passenger compartment pressure when the vehicle is not moving.
Another problem often causing passenger discomfort in a vehicle is the sudden pressure increase in the passenger compartment when a door is closed. This sudden pressure increase may occur when a passenger is in a vehicle with all the windows up and only one door open. When the door that was open is closed, it acts as an air ram, forcing air into the passenger compartment. With this influx of air into the passenger compartment comes a sharp increase of pressure in the compartment. This phenomenon may cause passenger discomfort.